The following explains why the massively multitasking brain has a limitation of paying attention to only one task at a time.

Even when LB can process multiple tasks parallelly, it can only process a dialogue with VP serially, because VP is a serial processor, which limits it to process only one task at a time.

Even with one task limitation of VP, mind can attend to more than one task at a time, known as multitasking (a common example being multiple tasks involved in driving a car). Besides the task it is attending to, other tasks VP processes are repetitive or condition based repetitive tasks, which are executed by LB
(as explained under title Repetitive Interactions and Condition Based Repetitive Interactions), which recalls them from memory, and thus, don't need attention (i.e. involvement of VP), other than their initiation. While processing more than one task at a time that need attention, the system initiates each of them using VP, switches them to autopilot using LB Monologue
(more under title LB Monologue), which is capable of parallel processing, puts them on standby using VP standby mode
(more under title How Interactions are Filtered) and initiates another task that needs attention using VP.

While switching attention between tasks, it seems that all tasks are available
simultaneously. The reason for the same is that the attention, i.e. subject of
VP-LB dialogue which is processing the task being considered, switches to the
other task without any delay (also mentioned under title Awareness), offering the effect of what is known as "refrigerator light phenomenon". The refrigerator light phenomenon is the feeling of light being always on, even when the refrigerator door is closed, which is when a limit switch turns it off. The effect it produces is like when communicating with a group of people, you switch from one person to the other, while all of them are available for communication as and when needed, giving the effect of simultaneity.